Existing microwave antennas include a wide variety of configurations for various applications, such as satellite reception, remote broadcasting, or military communication. The desirable characteristics of low cost, light-weight, low profile and mass producibility are provided in general by printed circuit antennas. The simplest forms of printed circuit antennas are microstrip antennas wherein flat conductive elements are spaced from a single essentially continuous ground element by a dielectric sheet of uniform thickness. An example of a microstrip antenna is disclosed in U.S. Pat. No. 3,995,277 to Olyphant.
The antennas are designed in an array and may be used for communication systems such as identification of friend/foe (IFF) systems, personal communication service (PCS) systems, satellite communication systems, and aerospace systems, which require such characteristics as low cost, light weight, low profile, and low sidelobes.
The bandwidth and directivity capabilities of such antennas, however, can be limiting for certain applications. While the use of electromagnetically coupled microstrip patch pairs can increase bandwidth, obtaining this benefit presents significant design challenges, particularly where maintenance of a low profile and broad beam width is desirable. Also, the use of an array of microstrip patches can improve directivity by providing a predetermined scan angle. However, utilizing an array of microstrip patches presents a dilemma. The scan angle can be increased if the array elements are spaced closer together, but closer spacing can increase undesirable coupling between antenna elements thereby degrading performance.
Furthermore, while a microstrip patch antenna is advantageous in applications requiring a conformal configuration, e.g. in aerospace systems, mounting the antenna presents challenges with respect to the manner in which it is fed such that conformality and satisfactory radiation coverage and directivity are maintained and losses to surrounding surfaces are reduced. More specifically, increasing the bandwidth of a phased array antenna with a wide scan angle is conventionally achieved by dividing the frequency range into multiple bands.
One example of such an antenna is disclosed in U.S. Pat. No. 5,485,167 to Wong et al. This antenna includes several pairs of dipole pair arrays each tuned to a different frequency band and stacked relative to each other along the transmission/reception direction. The highest frequency array is in front of the next lowest frequency array and so forth.
This approach may result in a considerable increase in the size and weight of the antenna while creating a Radio Frequency (RF) interface problem. Another approach is to use gimbals to mechanically obtain the required scan angle. Yet, here again, this approach may increase the size and weight of the antenna and result in a slower response time.
Harris Current Sheet Array (CSA) technology represents the state of the art in broadband, low profile antenna technology. For example, U.S. Pat. No. 6,512,487 to Taylor et al. is directed to a phased array antenna with a wide frequency bandwidth and a wide scan angle by utilizing tightly packed dipole antenna elements with large mutual capacitive coupling. The antenna of Taylor et al. makes use of, and increases, mutual coupling between the closely spaced dipole antenna elements to prevent grating lobes and achieve the wide bandwidth.
A slot version of the CSA has many advantages over the dipole version including the ability to produce vertical polarization at horizon, metal aperture coincident with external ground plane, reduced scattering, and stable phase center at aperture. Conformal aircraft antennas frequently require a slot type pattern, but the dipole CSA does not address these applications. Analysis and measurements have shown that the dipole CSA cannot meet requirements for vertical polarized energy at the horizon. The Dipole CSA is also limited in wide angle scan performance due to dipole-like element pattern over a ground plane.
A general implementation of a phased array may be capable of focusing the energy from all antenna elements to any desired point in space. Phased array antennas may typically have the elements arranged in a rectangular grid and be capable of focusing the antenna array pattern from broadside to the array to angles nearing 50 degrees off of broadside without difficulty. Scanning the array to angles exceeding 50 degrees becomes increasingly more difficult. In some applications, however, it may be desirable to operate an array in an endfire mode, which directs the radiation along the axis of the array at a scan angle of 0 degrees, corresponding to 90 degrees from broadside.
Endfire operation is a difficult mode in which to use a phased array. An antenna array's ability to scan to angles approaching endfire may include several problems, and traditional designs of antenna arrays used to scan in the endfire direction may need specialized antenna elements with limited fields-of-view (FOV). Furthermore, there may be a need for a broadband conformal endfire array that can be applied to a specific structure such as a tube or cylinder.